The present invention relates to a method for detecting or analyzing an agricultural chemical possibly remaining in farm products, for instance, grains such as rice; vegetables; and fruits and more particularly to a method for analyzing such residual agricultural chemicals present in farm products, which permits the detection of, in particular, new series of carbamate type agricultural chemicals.
The inventors of this invention have previously proposed a method for detecting residual agricultural chemicals, which permits the simultaneous detection of an organophosphorus agricultural chemical and a carbamate type agricultural chemical (see the Patent Reference 1 given later).
This method would allow, with ease, the division of the agricultural chemicals remaining in a sample to be analyzed into organophosphorus type ones and carbamate type ones and the simultaneous detection of these different kind agricultural chemicals and the method comprises the steps of admixing the sample with a mixed solution containing a carboxyl esterase and a choline esterase to thus react them; hydrolyzing a substrate specific to the carboxyl esterase and a substrate specific to the choline esterase using the solution obtained after the foregoing reaction to thus make the hydrolyzates of these substrates develop colors, while adding, to the system, a color former or a coupler to the hydrolyzates or selecting the foregoing substrates in such a manner that they can form hydrolyzates capable of developing colors in themselves; and simultaneously detecting spectral changes of a plurality of light rays having different wavelengths.
Because of the foregoing constitution, this method permits, with ease, the simultaneous discrimination and detection of an organophosphorus agricultural chemical and a carbamate type agricultural chemical possibly present in a sample to be analyzed (or a test sample). More specifically, this method is characterized by hydrolyzing a substrate specific to the carboxyl esterase and a substrate specific to the choline esterase; simultaneously detecting absorption spectra ascribable to the decomposition of these two kinds of substrates using a spectrophotometer which can simultaneously irradiate the hydrolyzed system with a plurality of light rays having different wavelengths and which permits the detection of any spectral change of the system with the elapse of time; calculating the reaction rates of each enzyme-catalyzed reaction according to an appropriate arithmetic treatment; and thus discriminatively detecting the organophosphorus agricultural chemical and the carbamate type agricultural chemical present in the sample on the basis of the changes in the reaction rates thus determined.
The foregoing method permits the discrimination and detection of a group of organophosphorus agricultural chemicals and that of carbamate type ones, but the precision of the analysis would be reduced depending on the kinds of agricultural chemicals and some of carbamate type ones hardly undergo inhibition of enzymes and this would make the precision of the analysis insufficient.
In this connection, it has been known that the carbamate type insecticides show their insecticidal action through the reduction of the action of a choline esterase which governs or manages the nervous systems of injurious insects. The carbamate type insecticides registered long ago suffer from such a problem that they are excellent in the fast-acting properties, but they are insufficient in the long-acting properties. Moreover, it had been desired for the improvement of the foregoing carbamate type insecticides in the latter half of 1970s because of the development or emergence of bacteria resistant to these insecticides in the bodies of injurious insects to be controlled. Under such circumstances, there have been registered, as new series of carbamate type agricultural chemicals, the following six components: carbofuran analogues (bendiocarb), carbofuran derivatives (carbosulfan, furathiocarb and benfuracarb) and methomyl derivatives (thiodicarb and alanicarb) since 1980s while taking the foregoing problems into consideration.
The foregoing new series of carbamate type agricultural chemicals except for bendiocarb are agricultural chemicals which are so designed that each of them is converted into carbofuran or methomyl having a greater activity of inhibiting the choline esterase activity due to the natural metabolic action after the spray thereof on farm products and then they gradually show their efficacy because of the formation of such effective components.
The carbofuran derivatives among the foregoing new series of carbamate type agricultural chemicals have structures (carbofuran-S—R) in which carbofuran having a strong activity of inhibiting choline esterase activity is linked to the other constituent molecule through a sulfide bond and the methomyl derivatives likewise have structures similar thereto (methomyl-S—R). These agricultural chemicals are ones which are so designed that they suffer natural metabolic actions after the spray thereof on farm products and as a result, they are gradually converted into carbofuran or methomyl, to thus show their insecticidal actions. In other words, the carbofuran derivatives and the methomyl derivatives show only weak abilities to inhibit the choline esterase and accordingly, they have never provided any satisfactory detection sensitivity with respect to the standard level of the residual agricultural chemical acceptable for farm products.
Patent Reference 1: Japanese Un-Examined Patent Publication 2003-298